lecture slides - philadelphia university 1 ch7… · shaft materials deflection primarily...

Chapter 7

Shafts and Shaft

Components

Lecture Slides

The McGraw-Hill Companies © 2012

Chapter Outline

Shigley’s Mechanical Engineering Design

Shaft Design

Material Selection

Geometric Layout

Stress and strength

◦ Static strength

◦ Fatigue strength

Deflection and rigidity

◦ Bending deflection

◦ Torsional deflection

◦ Slope at bearings and shaft-supported elements

◦ Shear deflection due to transverse loading of short shafts

Vibration due to natural frequency


Shaft Materials

Deflection primarily controlled by geometry, not material

Stress controlled by geometry, not material

Strength controlled by material property


Shaft Materials

Shafts are commonly made from low carbon, CD or HR steel,

such as ANSI 1020–1050 steels.

Fatigue properties don’t usually benefit much from high alloy

content and heat treatment.

Surface hardening usually only used when the shaft is being

used as a bearing surface.


Shaft Materials

Cold drawn steel typical for d < 3 in.

HR steel common for larger sizes. Should be machined all over.

Low production quantities

◦ Lathe machining is typical

◦ Minimum material removal may be design goal

High production quantities

◦ Forming or casting is common

◦ Minimum material may be design goal


Shaft Layout

Issues to consider for

shaft layout

◦ Axial layout of

components

◦ Supporting axial

loads

◦ Providing for torque

transmission

◦ Assembly and

Disassembly


Fig. 7-1

Axial Layout of Components


Fig. 7-2

Supporting Axial Loads

Axial loads must be supported through a bearing to the frame.

Generally best for only one bearing to carry axial load to shoulder

Allows greater tolerances and prevents binding


Fig. 7-3 Fig. 7-4

Providing for Torque Transmission

Common means of transferring torque to shaft

◦ Keys

◦ Splines

◦ Setscrews

◦ Pins

◦ Press or shrink fits

◦ Tapered fits

Keys are one of the most effective

◦ Slip fit of component onto shaft for easy assembly

◦ Positive angular orientation of component

◦ Can design key to be weakest link to fail in case of overload


Assembly and Disassembly


Fig. 7-5

Fig. 7-6

Assembly and Disassembly


Fig. 7-7

Fig. 7-8

Shaft Design for Stress

Stresses are only evaluated at critical locations

Critical locations are usually

◦ On the outer surface

◦ Where the bending moment is large

◦ Where the torque is present

◦ Where stress concentrations exist


Shaft Stresses

Standard stress equations can be customized for shafts for

convenience

Axial loads are generally small and constant, so will be ignored

in this section

Standard alternating and midrange stresses

Customized for round shafts


Shaft Stresses

Combine stresses into von Mises stresses


Shaft Stresses

Substitute von Mises stresses into failure criteria equation. For

example, using modified Goodman line,

Solving for d is convenient for design purposes


Shaft Stresses

Similar approach can be taken with any of the fatigue failure

criteria

Equations are referred to by referencing both the Distortion

Energy method of combining stresses and the fatigue failure

locus name. For example, DE-Goodman, DE-Gerber, etc.

In analysis situation, can either use these customized equations

for factor of safety, or can use standard approach from Ch. 6.

In design situation, customized equations for d are much more

convenient.


Shaft Stresses

DE-Gerber


where

Shaft Stresses

DE-ASME Elliptic

DE-Soderberg


Shaft Stresses for Rotating Shaft

For rotating shaft with steady bending and torsion

◦ Bending stress is completely reversed, since a stress element

on the surface cycles from equal tension to compression

during each rotation

◦ Torsional stress is steady

◦ Previous equations simplify with Mm and Ta equal to 0


Checking for Yielding in Shafts

Always necessary to consider static failure, even in fatigue

situation

Soderberg criteria inherently guards against yielding

ASME-Elliptic criteria takes yielding into account, but is not

entirely conservative

Gerber and modified Goodman criteria require specific check for

yielding


Checking for Yielding in Shafts

Use von Mises maximum stress to check for yielding,

Alternate simple check is to obtain conservative estimate of s'max

by summing s'a and s'm


max a ms s s

Example 7-1


Estimating Stress Concentrations

Stress analysis for shafts is highly dependent on stress

concentrations.

Stress concentrations depend on size specifications, which are

not known the first time through a design process.

Standard shaft elements such as shoulders and keys have

standard proportions, making it possible to estimate stress

concentrations factors before determining actual sizes.


Estimating Stress Concentrations


Reducing Stress Concentration at Shoulder Fillet

Bearings often require relatively sharp fillet radius at shoulder

If such a shoulder is the location of the critical stress, some

manufacturing techniques are available to reduce the stress

concentration

(a) Large radius undercut into shoulder

(b) Large radius relief groove into back of shoulder

(c) Large radius relief groove into small diameter of shaft


Fig. 7-9

Example 7-2


Example 7-2


Fig. 7-10

Example 7-2


Deflection Considerations

Deflection analysis at a single point of interest requires complete

geometry information for the entire shaft.

For this reason, a common approach is to size critical locations

for stress, then fill in reasonable size estimates for other

locations, then perform deflection analysis.

Deflection of the shaft, both linear and angular, should be

checked at gears and bearings.



Allowable deflections at components will depend on the

component manufacturer’s specifications.

Typical ranges are given in Table 7–2



Deflection analysis is straightforward, but lengthy and tedious to

carry out manually.

Each point of interest requires entirely new deflection analysis.

Consequently, shaft deflection analysis is almost always done

with the assistance of software.

Options include specialized shaft software, general beam

deflection software, and finite element analysis software.


Example 7-3


Fig. 7-10

Example 7-3


Example 7-3


Fig. 7-11

Example 7-3


Adjusting Diameters for Allowable Deflections

If any deflection is larger than allowed, since I is proportional to

d4, a new diameter can be found from

Similarly, for slopes,

Determine the largest dnew/dold ratio, then multiply all diameters

by this ratio.

The tight constraint will be just right, and the others will be

loose.


Example 7-4


Angular Deflection of Shafts

For stepped shaft with individual cylinder length li and torque Ti,

the angular deflection can be estimated from

For constant torque throughout homogeneous material

Experimental evidence shows that these equations slightly

underestimate the angular deflection.

Torsional stiffness of a stepped shaft is


Critical Speeds for Shafts

A shaft with mass has a critical speed at which its deflections

become unstable.

Components attached to the shaft have an even lower critical

speed than the shaft.

Designers should ensure that the lowest critical speed is at least

twice the operating speed.



For a simply supported shaft of uniform diameter, the first

critical speed is

For an ensemble of attachments, Rayleigh’s method for lumped

masses gives



Eq. (7–23) can be applied to the shaft itself by partitioning the

shaft into segments.


Fig. 7–12


An influence coefficient is the transverse deflection at location I

due to a unit load at location j.

From Table A–9–6 for a simply supported beam with a single

unit load

Shigley’s Mechanical Engineering Design Fig. 7–13


Taking for example a simply supported shaft with three loads, the

deflections corresponding to the location of each load is

If the forces are due only to centrifugal force due to the shaft mass,

Rearranging,



Non-trivial solutions to this set of simultaneous equations will

exist when its determinant equals zero.

Expanding the determinant,

Eq. (7–27) can be written in terms of its three roots as



Comparing Eqs. (7–27) and (7–28),

Define wii as the critical speed if mi is acting alone.

From Eq. (7–29),

Thus, Eq. (7–29) can be rewritten as


2

1i ii

ii

mw


Note that

The first critical speed can be approximated from Eq. (7–30) as

Extending this idea to an n-body shaft, we obtain Dunkerley’s

equation,



Since Dunkerley’s equation has loads appearing in the equation, it

follows that if each load could be placed at some convenient

location transformed into an equivalent load, then the critical speed

of an array of loads could be found by summing the equivalent

loads, all placed at a single convenient location.

For the load at station 1, placed at the center of the span, the

equivalent load is found from


Example 7-5


Setscrews

Setscrews resist axial and rotational motion

They apply a compressive force to create friction

The tip of the set screw may also provide a slight penetration

Various tips are available


Fig. 7–15

Setscrews

Resistance to axial motion of collar or hub relative to shaft is called holding power

Typical values listed in Table 7–4 apply to axial and torsional resistance

Typical factors of safety are 1.5 to 2.0 for static, and 4 to 8 for dynamic loads

Length should be about half the shaft diameter


Keys and Pins

Used to secure

rotating elements and

to transmit torque


Fig. 7–16

Tapered Pins

Taper pins are sized by diameter at large end

Small end diameter is

Table 7–5 shows some standard sizes in inches

Shigley’s Mechanical Engineering Design Table 7–5

Keys

Keys come in

standard square

and rectangular

sizes

Shaft diameter

determines key

size

Shigley’s Mechanical Engineering Design Table 7–6

Keys

Failure of keys is by either direct shear or bearing stress

Key length is designed to provide desired factor of safety

Factor of safety should not be excessive, so the inexpensive key

is the weak link

Key length is limited to hub length

Key length should not exceed 1.5 times shaft diameter to avoid

problems from twisting

Multiple keys may be used to carry greater torque, typically

oriented 90º from one another

Stock key material is typically low carbon cold-rolled steel,

with dimensions slightly under the nominal dimensions to

easily fit end-milled keyway

A setscrew is sometimes used with a key for axial positioning,

and to minimize rotational backlash


Gib-head Key

Gib-head key is tapered so that when firmly driven it prevents

axial motion

Head makes removal easy

Projection of head may be hazardous


Fig. 7–17

Woodruff Key

Woodruff keys have deeper penetration

Useful for smaller shafts to prevent key from rolling

When used near a shoulder, the keyway stress concentration

interferes less with shoulder than square keyway


Fig. 7–17

Woodruff Key


Stress Concentration Factors for Keys

For keyseats cut by standard end-mill cutters, with a ratio of

r/d = 0.02, Peterson’s charts give

◦ Kt = 2.14 for bending

◦ Kt = 2.62 for torsion without the key in place

◦ Kt = 3.0 for torsion with the key in place

Keeping the end of the keyseat at least a distance of d/10 from

the shoulder fillet will prevent the two stress concentrations

from combining.


Example 7-6


Fig. 7–19

Example 7-6


Retaining Rings

Retaining rings are often used instead of a shoulder to provide

axial positioning


Fig. 7–18

Retaining Rings

Retaining ring must seat well in bottom of groove to support

axial loads against the sides of the groove.

This requires sharp radius in bottom of groove.

Stress concentrations for flat-bottomed grooves are available in

Table A–15–16 and A–15–17.

Typical stress concentration factors are high, around 5 for

bending and axial, and 3 for torsion


Limits and Fits

Shaft diameters need to be sized to “fit” the shaft components

(e.g. gears, bearings, etc.)

Need ease of assembly

Need minimum slop

May need to transmit torque through press fit

Shaft design requires only nominal shaft diameters

Precise dimensions, including tolerances, are necessary to

finalize design


Tolerances

Close tolerances generally

increase cost

◦ Require additional

processing steps

◦ Require additional

inspection

◦ Require machines with

lower production rates


Fig. 1–2

Standards for Limits and Fits

Two standards for limits and fits in use in United States

◦ U.S. Customary (1967)

Metric (1978)

Metric version will be presented, with a set of inch conversions


Nomenclature for Cylindrical Fit

Upper case letters

refer to hole

Lower case letters

refer to shaft

Basic size is the

nominal diameter and

is same for both parts,

D=d

Tolerance is the

difference between

maximum and

minimum size

Deviation is the

difference between a

size and the basic size Shigley’s Mechanical Engineering Design

Fig. 7–20

Tolerance Grade Number

Tolerance is the difference between maximum and minimum size

International tolerance grade numbers designate groups of

tolerances such that the tolerances for a particular IT number

have the same relative level of accuracy but vary depending on

the basic size

IT grades range from IT0 to IT16, but only IT6 to IT11 are

generally needed

Specifications for IT grades are listed in Table A–11 for metric

series and A–13 for inch series


Tolerance Grades – Metric Series


Table A–11

Tolerance Grades – Inch Series


Table A–13

Deviations

Deviation is the algebraic difference between a size and the basic

size

Upper deviation is the algebraic difference between the maximum

limit and the basic size

Lower deviation is the algebraic difference between the minimum

limit and the basic size

Fundamental deviation is either the upper or lower deviation that

is closer to the basic size

Letter codes are used to designate a similar level of clearance or

interference for different basic sizes


Fundamental Deviation Letter Codes

Shafts with clearance fits

◦ Letter codes c, d, f, g, and h

◦ Upper deviation = fundamental deviation

◦ Lower deviation = upper deviation – tolerance grade

Shafts with transition or interference fits

◦ Letter codes k, n, p, s, and u

◦ Lower deviation = fundamental deviation

◦ Upper deviation = lower deviation + tolerance grade

Hole

◦ The standard is a hole based standard, so letter code H is always used for the hole

◦ Lower deviation = 0 (Therefore Dmin = 0)

◦ Upper deviation = tolerance grade

Fundamental deviations for letter codes are shown in Table A–12 for metric series and A–14 for inch series


Fundamental Deviations – Metric series

Shigley’s Mechanical Engineering Design Table A–12

Fundamental Deviations – Inch series


Table A–14

Specification of Fit

A particular fit is specified by giving the basic size followed by

letter code and IT grades for hole and shaft.

For example, a sliding fit of a nominally 32 mm diameter shaft

and hub would be specified as 32H7/g6

This indicates

◦ 32 mm basic size

◦ hole with IT grade of 7 (look up tolerance DD in Table A–11)

◦ shaft with fundamental deviation specified by letter code g

(look up fundamental deviation F in Table A–12)

◦ shaft with IT grade of 6 (look up tolerance Dd in Table A–11)

Appropriate letter codes and IT grades for common fits are given

in Table 7–9


Description of Preferred Fits (Clearance)


Table 7–9

Description of Preferred Fits (Transition & Interference)


Table 7–9

Procedure to Size for Specified Fit

Select description of desired fit from Table 7–9.

Obtain letter codes and IT grades from symbol for desired fit

from Table 7–9

Use Table A–11 (metric) or A–13 (inch) with IT grade numbers

to obtain DD for hole and Dd for shaft

Use Table A–12 (metric) or A–14 (inch) with shaft letter code to

obtain F for shaft

For hole

For shafts with clearance fits c, d, f, g, and h

For shafts with interference fits k, n, p, s, and u


Example 7-7


Example 7-8


Stress in Interference Fits

Interference fit generates pressure at interface

Need to ensure stresses are acceptable

Treat shaft as cylinder with uniform external pressure

Treat hub as hollow cylinder with uniform internal pressure

These situations were developed in Ch. 3 and will be adapted

here



The pressure at the interface, from Eq. (3–56) converted into

terms of diameters,

If both members are of the same material,

is diametral interference

Taking into account the tolerances,



From Eqs. (3–58) and (3–59), with radii converted to diameters,

the tangential stresses at the interface are

The radial stresses at the interface are simply

The tangential and radial stresses are orthogonal and can be

combined using a failure theory


Torque Transmission from Interference Fit

Estimate the torque that can be transmitted through interference

fit by friction analysis at interface

Use the minimum interference to determine the minimum

pressure to find the maximum torque that the joint should be

expected to transmit.


lecture slides - philadelphia university 1 ch7… · shaft materials deflection primarily...

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